The most commonly used immobilization matrix is algal polysaccharide alginate, cross-linked with calcium ions as described in FEBS Lett. 103: 93-97 (1979) and 122: 312-316 (1980), Plant Cell Rep. 5: 302-305 (1986), and Appl. Microbiol. Biotechnol. 30: 475-481 (1989). Other methods include entrapment with polyurethane foam, described in Biotechnol. Bioeng. 33: 293-299 (1989), 35: 660-667 (1990), Appl. Microbiol. Biotechnol. 33: 36-42 (1990), 37: 397-403 (1991) and 35: 382-392 (1991).
The use of porous and amorphous sol-gel derived silica for entrapment of viable cells has been described in cases of non-plant cells, reported in J. Biotechnol. 30: 197 (1993), J. Ceram. Soc. Jpn. 100: 426 (1992), Biochim. Biophys. Acta 276: 323 (1972), Chemistry of Materials 6: 1605-1613 (1994) and Angew, Chem, Int. Engl. 34: 301-303 (1995). These latter cannot be considered as methods actually suitable for applications to higher plant cells, which are severely poisoned under the experimental conditions reported for immobilization of bacteria and yeast cells. As for processes applied to, and claimed for, plant cell immobilization, they entail some problems: first of all, processes based on simple adhesion to the surface cannot properly be considered as immobilization, since cell reproduction and the increase in biological mass unavoidably cause release of cells in solution.
Polyurethane foaming matrices may cause severe transport limitations to and from immobilized cells. One important drawback of these host matrices is their poor mechanical stiffness, so that prolonged use for industrial production in practice does not appear feasible.
Immobilization in alginate beads allows direct contact of cells with the gel matrix, so that the cells are inevitably subjected to a high concentration of a variety of ions and organic compounds, causing negative physiological effects.